Instruments
There is such a big variety of musical instruments on all islands that it is nearly impossible to give an overview.
Of course the Gamelan ensemble is essential, but also several Arabian influencies, which brought the Oud lute to Java and Sumatra, where it is called "Gambus".  Ensemble music is in the center of the old and slow court music in Java, as well as in dance and theatre music. The music of the shadow theatre is faster and more percussive, while the musical traditions of Bali are a bit outstanding as there was never any Islamic occupation.
On Bali we find for example the very fast "Jegog" bamboo xylofone ensembles and the beautiful dance traditions.
Another outstanding and least explored region is Borneo, where several ethnics keep their own traditions and instruments.
While most of the smaller islands still remain unresearched, Indonesias map holds a lot of musical treasures which still surprise us, as we can see on the wonderful blog of Palmer Keen

Here ar some instruments in my collection:

From Java+Sumatra:
o The bamboo flutes Suling
o The gong set Bonang
o The bamboo shaker Angklung
o The drum Kendang
o The violin Rebab
o The lute Rabab
o The lute Kecapi/Hasalpi
o The reed flute Toleat

From Sulawesi:
o The bamboo flute Suling Lembang
o The bamboo flutes Suling Makassar
o The boatlute Kecapi
o The bamboo flute M'basing
o The bamboo flute Lalove
o The bamboo zither Sattung
o The reed Keke
o The shawm Puik-puik
o The bamboo buzzer Gongga Lima
o The mouth harp Gongga Lawe
o The mouth harp Yori
o The mouth harp Karombi

From Papua:
o The mouth harp Susap

From Kalimantan:
o The bamboo ocarina Lontu

From Timor:
o The bamboo tube zither Sasando